This invention relates to a circuit for driving a transmission line.
A large switcher may have several hundred inputs and only one output. The purpose of the switcher is to allow a single input to be selected and connected to the output, while all the other inputs are isolated from the output.
It is conventional to implement a large switcher having n*k inputs and only one output by using k crosspoint modules each having n inputs and one output. Each of these modules can be viewed conceptually as an array of n parallel input lines, a single output line extending perpendicular to the input lines, and n crosspoint elements connected between the output line and the input lines respectively. The crosspoint elements are switches, each of which is able to connect a single input line of the crosspoint module to the output line thereof. A configuration controller is used to select any one of the crosspoint elements, and the selected crosspoint element is rendered conductive while the other crosspoint elements remain non-conductive. The input line associated with the selected crosspoint element is thereby connected to the output bus and provides the switcher output. It will be appreciated that the input and output lines of the switcher might each be a single conductor for carrying single-ended signals, or two conductors for carrying differential signals.
The output lines of the crosspoint modules may be connected through an output bus to an output module that provides a switcher output and buffers the output bus.
In order to apply this output bus architecture to a switcher used for switching high speed digital signals, e.g. data signals at 300 Mb/s or higher, the bus must be treated as a transmission line, i.e. a conduction path that has a uniform characteristic impedance along its length. If an unterminated stub projects from the conduction path, this may result in the path not having a uniform characteristic impedance. The allowable length of a stub that can project from the conduction path without causing an unacceptable variation in characteristic impedance decreases as the signal frequency increases. For a digital data signal at 300 Mb/s, the maximum acceptable length of an unterminated stub is about 1-2 cm. Restrictions inherent in the construction of a large switcher generally necessitate that the crosspoint modules be at a distance of 10 cm or more from the output bus, and the conductors extending from the crosspoint modules to the bus constitute unterminated stubs. These unterminated stubs preclude use of the output bus architecture with high speed digital signals, and therefore switchers designed for use with high speed digital signals generally use multiplexers to select the crosspoint module that is to provide the switcher's output.
The output lines of the crosspoint modules are connected to their respective multiplexers through output drivers. The output drivers are normally driven continuously even though only one multiplexer at a time provides the switcher's output signal. The total power consumed by the k output drivers is then approximately k times the power needed to provide the output signal of the selected crosspoint module.